1. Field of the Invention
The present invention relates to a developing apparatus for an image reproduction such as electrophotographic apparatus, facsimile and the like.
2. Description of the Prior Art
In electrophotographic apparatus, for example, a reproduction image is obtained in such a manner that a uniformly electrostatically charged photoreceptor is imagewise exposed to the light reflected from a document to be reproduced and to form thereonto an electrostatic latent image corresponding to the document, the electrostatic latent image being developed by a developer to a visible image, which is then fixed as it is, or is transferred onto a recording paper and then fixed. The developer is stored in a developer container, inside which is disposed a cylindrical sleeve consisting of a nonmagnetic material which is partially immersed in the developer and is in close proximity to the surface of the photosensitive body. Inside the sleeve a plurality of magnets are disposed with a given space apart from each other in close proximity to the interior of the sleeve, and the sleeve and the group of the magnets are mounted so that they are relatively rotatable. The sleeve and the group of the magnets together form a developing means, and with the rotation of either, said sleeve or the group of the magnets, the developer inside the said container is transported along the surface of the sleeve and is then brought into contact with the surface of the photoreceptor, thereby forming a visible image on said photoreceptor. The developer, at the time of being transported from the developer container, is held to a given height by a regulating plate extending axially along and substantially adjacent the sleeve surface. Such type of developing apparatus is known as magnetic brush type developing apparatus.
Most electrophotographic copying apparatus now widely used employs a developer composed of a magnetic powder called "carrier" and colored resin particles called a "toner", and such a developer is called a "two-component developer". In contrast to this, a different developer consisting of toner alone, the so-called one-component developer, has emerged, which consists of a magnetic toner, a resin containing a magnetic material and, if needed, a coloring agent. This developer has many advantages such that there occurs no such change in the reproduced image density according to the consumption of toner as in the case of the two-component developer, nor need of toner replenishment according to such change, nor deterioration of the carrier, thus requiring no density detector nor special toner replenishing means, nor change of carrier, whereupon the developing apparatus itself can be of a compact type. For this reason, the one-component developer is gradually being made a practical reality.
A two-component developer is normally composed of carrier particles consisting of a magnetic material with a large particle diameter of from 50 to 200.mu. and fine toner particles with a particle diameter of from 5 to 20.mu.. As the magnetic material for the carrier, iron powder is used. The mixing proportion of the toner to the carrier is about 2 to 10% by weight.
In such a two-component developer, with the rotation of a developing means provided therein with a group of magnets, the iron powder carrier is transported by the action of the foregoing group of magnets, but the toner, because of being strongly electrically attracted to and about the iron powder carrier, is transported together with the carrier, so that the toner is only slightly scattered in the course of being transported, a little flowing out through a small opening because of the large weight of the carrier with the large particle diameter.
In contrast to this, the one-component developer has no such large diameter particles as corresponding to that of the foregoing two-component developer and is really a toner having the particle diameter of from 5 to 20.mu..
Where such a one-component developer is used in a magnetic brush type developing apparatus, the developer is scattered and flies high like a mist, which contaminates the inside or outside of the apparatus or intrudes into the rotary bearing of the sleeve or the group of the magnets to increase the load of their rotation, thus sometimes scoring or burning the supporting shaft or bearing. The one-component developer, because of having a small content ratio of the magnetic material component, is fairly weak in its adherence to the sleeve. Therefore, although being compensated by the increased magnetic force of the group of the magnets, as strong adherence as that of two-component developers cannot be obtained, so that the thickness of the developer layer formed on the sleeve becomes as thin as from 1 to 2 mm, thus necessitating a fairly small gap between the regulating plate and the sleeve. Since the magnetic forces which retains the magnetic toner on the sleeve have a tangential component at the sleeve-axial magnet holding end and the magnetic force is small, the layer of developer transported being height-controlled through the small gap spreads out and rises as ears at both ends of the control plate, thus having a bad influence upon the subsequent developing effects.
For this reason, U.S. Pat. No. 4,292,922, issued Oct. 6, 1981 to co-inventors Yoshio Yamazaki, Ninichi Kamogawa and Yasuyuki Iwai, describes the prevention of such large rise of the developer by providing a partition plate and a partition auxiliary plate at both axial ends on the sleeve, and a regulating plate. However, we have found as a result of our experiments that the developer, after being transported out through the slight gap between the surface of the sleeve and these partition plates, partition auxiliary plates or regulating plates, rises again during the period until it reaches the developing section. The rise of the developer, although not so large (only 0.25 to 0.5 mm higher than the average portion) during the time of being transported on the rotating sleeve, becomes several times higher (becoming 1 to 2 mm higher than the average portion) at the point of earing when it has arrived at the developing section, thus coming into strong contact with the photosensitive body, accelerating the deterioration of the photoreceptor, bringing about a blank area or fog on the image, and/or causing the deterioration of the image quality.